From Space, Earth Twinkles

This photo, taken by the EPIC camera on DSCOVR, shows a glint or sparkle of light coming from Earth’s atmosphere over South America. The glint shows in three different colors because the camera takes the red, green, and blue images several minutes apart. NASA image courtesy of the DSCOVR EPIC team, NASA GSFC, with Mike Carlowicz

NASA’s Deep Space Climate Observatory (DSCOVR), situated 1 million miles (1.6 million km) from Earth toward the sun, has discovered something about our planet no one expected: flashes of light reflecting into space, and we’re not talking lightning. The satellite keeps track of the solar wind and storms from the sun to provide timely space weather forecasts. It’s also equipped with an Earth Polychromatic Imaging Camera (EPIC) that measures the planet’s clouds, aerosols, ozone and acquires full-disk photos of Earth hourly you can view on the mission’s site. Over the past two years, EPIC has detected hundreds of flashes of light reflecting off the planet.

DSCOVR orbits around the L1 Lagrangian point (a stable spot) one million miles sunward of Earth. From here, the EPIC camera sees a fully illuminated Earth 24 hours a day.Credit: NOAA

In the months after the satellite launched, keen observers from outside NASA wrote to the science team, questioning the source of these lights. Alexander Marshak, DSCOVR deputy project scientist at NASA’s Goddard Space Flight Center, first noticed light flashes occasionally appearing over oceans as he looked through daily EPIC images. Marshak and colleagues did a little investigating and discovered that Carl Sagan reported similar flashes in 1993 when looking at photos taken by the Galileo spacecraft.

EPIC Observes Ice in Earth’s Atmosphere

Flashes of light off of the oceans are easily explained as mirror-like reflections of sunlight that reflect off the water directly back to the sensor. But when the scientists took another took a look at the Galileo images, they saw something Sagan and his colleagues missed: bright flashes over land. And those flashes appeared in the EPIC images as well. At first you might think, they might be glints from lakes or even rivers, but the location and size of the flashes didn’t match.

Could it be something in the sky? Marshak and colleagues conducted a yearlong series of experiments examining flash locations, light scattering and how light is absorbed by oxygen (to determine cloud heights) they detailed in a new paper published in Geophysical Letters. Their analysis confirmed that the cause of the distant flashes is sunlight reflecting off tiny platelets of ice floating nearly horizontally in the atmosphere.

The EPIC data also helped confirm that the flashes are coming from a high altitude, not simply water on the ground. Two channels on the instrument are designed to measure the height of clouds. According to the observations, familiar cirrus clouds — mare’s tails as they’re often called — 3 to 5 miles (5 to 8 km) up in the atmosphere appeared wherever the glints were located.

Cirrus clouds are thin and wispy and form between 16,500 and 45,000 feet high. Their great altitude means they’re composed of ice crystals unlike lower clouds made of liquid water droplets. Glints from the clouds have been detected a million miles away in space. Credit: Bob King

“The source of the flashes is definitely not on the ground,” Marshak said. “It is definitely ice, and most likely solar reflection off of horizontally oriented particles.”

Marshak is now investigating how common these horizontal ice particles are, and whether they might have a measurable effect on how much sunlight passes through the atmosphere. If so, the information would need to be included into computer models of how much heat is reaching and leaving Earth. Detecting starlight glint off exoplanets, if it can be done, may also help us tap into the composition of their atmospheres.